Variable coupling factor Directional Coupler

ABSTRACT

A variable coupling factor directional coupler having a variable aperture positioned between a transmission line section and a coupling conductor connected at either end to a center conductor of a pair of connectors. The coupling factor of an RF signal in the transmission line section to the coupling conductor may be adjusted by linear movement of a gap plate to open or close the aperture. Alternatively, the coupling conductor may be located within a slotted tube. As the slotted tube is rotated, the slotted portion of the tube opens or closes the aperture. The position of the inner conductor of the coupling conductor with respect to a grounded sidewall can be adjusted to change the coupled line impedance in order to optimize the coupler directivity.

BACKGROUND OF INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to directional couplers. Moreparticularly, the invention is concerned with a cost efficientdirectional coupler having a variable coupling factor.

[0003] 2. Description of Related Art

[0004] Directional couplers are useful for sampling and or measuring RFenergy. The directional characteristic of directional couplers allowsseparate measurement and or sampling of the forward and reflectedcomponents of RF energy traveling along, for example, a coaxial cable.The coupling factor is a measure of how much of the total RF energypresent in a main cable is coupled to an auxiliary cable, the remaindercontinuing along the main cable. Variable coupling factor functionalityallows the level of sampling and or measurement to be adjusted.

[0005] Mathematical models for the electrical interaction betweencoupled lines of unequal cross section and coupled coaxial lines inparticular are well known to those skilled in the art. Also, factorsinfluencing directivity in a directional coupler are known.

[0006] Common for usage in high power RF systems are directionalcouplers with loose coupling values (30-50 dB) between a main powercarrying line of large size (1⅝″ EIA to 8{fraction (3/16)}″ orwaveguide) and a small size coupled line feeding a monitor or feedbackcircuit (interconnected using, for example, type N or TNC connectors).

[0007] Couplers implemented with a variable rather than fixed couplingfactor have some advantages over fixed coupling factor couplers. Forexample, they can serve as a flexible test instrument and be field setfor specific applications. They are also useful in high power low VSWRsystems where monitoring forward power requires a low coupling factor inorder to protect the detector but also a higher coupling factor todetect a typically much lower reflected power. They are also useful in aproduction environment where a single assembly can be stocked andrapidly adjusted to a range of desired coupling factors.

[0008] The typical approach for loosely coupled mechanically adjustabledirectional couplers is to use an electrically short (less than onequarter wavelength) coupled line whose proximity to the main line can bevaried. By moving the coupled line closer to the main line the couplingis increased and by moving it farther away the coupling is decreased.The directivity of the coupler is then optimized for specific couplingvalues by rotating the coupled lines orientation with respect to themainline. Orientations of 30° to 60° are typical. This design approachrequires a coupled line assembly with two mechanical degrees of freedom(proximity and rotation) with respect to the mainline. The cost ofmanufacture of such an assembly may be relatively expensive. The factthat the coupled line is electrically short means that the couplingvalue is not flat over a broad frequency range, generally falling off at6 dB per octave.

[0009] Competition within the coupler industry has focused attention onreduction of coupler materials and manufacturing costs.

[0010] Therefore, it is an object of the invention to provide anapparatus that overcomes deficiencies in the prior art.

BRIEF DESCRIPTION OF DRAWINGS

[0011] The accompanying drawings, which are incorporated in andconstitute a part of this specification, illustrate embodiments of theinvention and, together with a general description of the inventiongiven above, and the detailed description of the embodiments givenbelow, serve to explain the principles of the invention.

[0012]FIG. 1 is an external isometric view showing a first embodiment ofthe invention.

[0013]FIG. 2 is an external side view of the embodiment of FIG. 1.

[0014]FIG. 3 is an external end view of the embodiment of FIG. 1.

[0015]FIG. 4 is a cut-away side view along the line AA of the embodimentshown in FIG. 1.

[0016]FIG. 5 is a top view, showing hidden lines, of the embodiment ofFIG. 1.

[0017]FIG. 6 is an exploded isometric view, from above, of theembodiment of FIGURE 1.

[0018]FIG. 7 is an exploded isometric view, from below, of theembodiment of FIG. 1.

[0019]FIG. 8 is an exploded isometric view of a second embodiment of theinvention.

[0020]FIG. 9 is an external top view of the embodiment of FIG. 8.

[0021]FIG. 10 is a cut-away side view, along the line AA, of FIG. 9.

[0022]FIG. 11a is a schematic cross section of two coaxial lines coupledthrough an aperture with three capacitances identified.

[0023]FIG. 11b is an equivalent circuit representation of the structureshown in FIG. 11a.

DETAILED DESCRIPTION

[0024] Referring to FIGS. 11a and 11 b, a preliminary description of theelectrical characteristics of coaxial lines coupled through an aperturefollows. “Ca” is the capacitance per unit length of the inner conductorof Line A coupled to ground. “Cb” is the capacitance per unit lengthbetween the inner conductor of Line B coupled to ground. “Cab” is thecapacitance per unit length between the inner conductors of Line A andLine B.

[0025] For practical couplers in high power systems, if Line B is themain line, “Cb” is fixed by the characteristic impedance thereof. Thisvalue is therefore preferably left unchanged in the coupler design. Asshown by the equivalent circuit representation of FIG. 11b, the couplingbetween the lines is proportional to “Cab”. Also, from coupled coaxialline electrical theory, the size of the aperture between the lines isdirectly proportional to “Cab”. The match and directivity of the couplerare complex functions of all three variables. If “Cb” is fixed and “Cab”is used to set the coupling factor, then “Ca” may be adjusted tooptimize the coupler match and directivity.

[0026] For purposes of illustration, a first embodiment of the inventionis shown in FIGS. 1-7. In the first embodiment, the variable couplingfactor directional coupler (VCFDC) 1 is configured for placement in-linewith a 1{fraction (5/8)} inch coaxial transmission line. Alternatively,the VCFDC 1 may be dimensioned for use with a coaxial transmission lineof any diameter, for example ¼ to 8{fraction (3/16)} inch diametercoaxial transmission line, cable or waveguide. In the first embodiment,each end 5 is shown configured for NF type connection. Alternatively,any form of connection, for example EIA flanges or other form of coaxialconnector, may also be used. The end(s) 5 are mounted to a body 10,having a center bore through which a center conductor 15 coaxiallypasses. The center conductor 15 may be supported by a dielectric orfree, held in a coaxial orientation with respect to the end(s) 5 and thebody 10 by the NF or other form of connection that links the VCFDC 1in-line with a transmission line coupled to either end 5 of the VCFDC 1.

[0027] The body 10 has a mounting surface with an aperture 20 thatextends through the body 10 to the dielectric space and the centerconductor 15. A connection plate 30 mates to the mounting surface,covering the aperture 20. A groove 35 (FIG. 7) on the underside of theconnection plate 30 is adapted to retain a gap plate 25 that is slidable(as shown in FIG. 6) within the groove 35 to open or close the aperture20 as desired. Alternatively, the groove 35 may be formed in the body10.

[0028] A pair of connectors 40, for example type N coaxial connectors,are mounted on a top side of the connection plate 30. A slot 50, alignedwith the aperture 20, formed on the under side of the connection plate30 extends between the connectors 40. The center conductors of eachconnector 40 are connected to either end of a coupling conductor 45 thatextends between the connector(s) 40 in the slot 50, spaced away from thesidewalls of the slot 50.

[0029] When the VCFDC 1 is connected in-line with a transmission line,RF signals propagating along the transmission line in the form ofelectric and or magnetic fields radiate through the aperture 20 andcouple with the coupling conductor 45. As the aperture 20 is opened orclosed by manipulating the gap plate 25, the electric and or magneticfields are variably exposed to or isolated from the coupling conductor45, allowing adjustment of the coupling to a desired coupling factor.With the aperture 20 completely open the VCFDC 1 has a maximum couplingvalue. When the gap plate 25 is used to close off the aperture 20, thecoupling factor is reduced. The maximum coupling factor is determined bythe length of the slot (one quarter wavelength or odd multiple thereoffor maximum coupling), the proximity of the conductors, the width of theslot and the width of the coupling conductor 45.

[0030] When a load 55 is attached to one of the connectors 40, thecoupling becomes directional, allowing separate measurement of forwardand reflected signals. Exchanging the load 55 to the other connector 40is a simple and fast way of changing the direction of coupling.Therefore, the VCFDC 1 is useful, for example, when calculating VSWR.The connectors 40 have oversized mounting holes in the form of connectorslot(s) 70. When the fasteners (not shown) used to mount the connectors40 are loosened the assembly consisting of the connectors 40 andcoupling conductor 45 can be moved laterally within the slot 50. Byadjusting the coupling conductors 45 position relative to the sidewallof the slot 50 the value “Ca” is increased or decreased. Using thisadjustment the directivity of the coupler may be optimized.

[0031] In alternative embodiments, the aperture 20 may be opened orclosed by, for example, an angular rather than linear adjustment. In asecond embodiment, as shown in FIGS. 8-10 (similar elements aresimilarly labeled), the aperture 20 is opened or closed by surroundingthe coupling conductor 45 with a slotted tube 60. The slotted tube 60may be electrically sealed by end plug(s) 65. As the slotted tube 60 isrotated, the coupling conductor 45 may be variably isolated from orexposed to RF energy, thereby adjusting the coupling factor.

[0032] In this embodiment, rather than using connector slot(s) 70, theconnection plate 30 has connection plate slot(s) 75 which allow theconnection plate 30, connector(s) 40 and coupling conductor 45 to movelaterally as a common assembly with respect to the slotted tube 60. Thismovement adjusts the position of the coupling conductor 45 with respectto the slotted tube 60, effectively changing the value of “Ca”. Thisadjustment can be used to optimize the coupler directivity for a givencoupling factor.

[0033] From the foregoing, it will be apparent that the presentinvention brings to the art a precision VCFDC 1 that does not requiremechanical linkages or precision threading to obtain variations incoupling factor. The simplified apparatus is therefore cost effective tomanufacture and less susceptible to mechanical wear. [Table Heading] 1variable coupling factor directional coupler 5 end 10 body 15 centerconductor 20 aperture 25 gap plate 30 connection plate 35 groove 40connector 45 coupling conductor 50 slot 55 load 60 slotted tube 65 endplugs 70 connector slot 75 connection plate slot

[0034] Where in the foregoing description reference has been made toratios, integers, components or modules having known equivalents thensuch equivalents are herein incorporated as if individually set forth.

[0035] While the present invention has been illustrated by thedescription of the embodiments thereof, and while the embodiments havebeen described in considerable detail, it is not the intention of theapplicant to restrict or in any way limit the scope of the appendedclaims to such detail. Additional advantages and modifications willreadily appear to those skilled in the art. Therefore, the invention inits broader aspects is not limited to the specific details,representative apparatus, methods, and illustrative examples shown anddescribed. Accordingly, departures may be made from such details withoutdeparture from the spirit or scope of applicant's general inventiveconcept. Further, it is to be appreciated that improvements and/ormodifications may be made thereto without departing from the scope orspirit of the present invention as defined by the following claims.

1. A variable coupling factor directional coupler, comprising: a bodyhaving a first side and a second side; a bore extending through the bodyfrom the first side to the second side; an elongated aperture in a sidewall of the bore; a coupling conductor proximate the aperture; and a gapplate located between the bore and the coupling conductor operable tocover a desired portion of the aperture.
 2. The apparatus of claim 1,wherein the coupling conductor is movable only within a plane tangentialto a longitudinal axis of the bore.
 3. The apparatus of claim 1, whereinthe coupling conductor is connected at a first end to a center conductorof a first coaxial connector and at a second end to a center conductorof a second coaxial connector.
 4. The apparatus of claim 3, wherein thefirst coaxial connector and the second coaxial connector are mounted toa connection plate connected to the body; the connection plate having aslot; the coupling conductor positioned in the slot.
 5. The apparatus ofclaim 4, wherein a mounting position of the first coaxial connector andthe second coaxial connector is adjustable via a plurality of connectorslots.
 6. The apparatus of claim 4, wherein a mounting position of theconnection plate onto the body is adjustable via a connection plateslot.
 7. The apparatus of claim 1, further including a first endconnected to the body coaxial with the bore on the first side and asecond end connected to the body coaxial with the bore on the secondside.
 8. The apparatus of claim 7, wherein the first end and the secondend are adapted for interconnection with a one of a coaxial cable, ahelically corrugated coaxial cable, and a waveguide.
 9. The apparatus ofclaim 4, wherein the coupling conductor is adapted to be adjustablelaterally with respect to a longitudinal axis of the bore, within theslot.
 10. A variable coupling factor directional coupler, comprising: abody having a first side and a second side; a first bore extendingthrough the body from the first side to the second side; a second boreextending through the body from the first side to the second side; aslotted tube mounted within the second bore; an elongated apertureinterconnecting the first bore with the second bore; and a couplingconductor positioned within an internal area of the slotted tube; andthe slotted tube rotatable to block the elongated aperture to a desireddegree, thereby selectively isolating the coupling conductor from thefirst bore.
 11. The apparatus of claim 10, wherein the couplingconductor is movable only within a plane tangential to a longitudinalaxis of the first bore.
 12. The apparatus of claim 10, wherein thecoupling conductor is connected at a first end to a center conductor ofa first coaxial connector and at a second end to a center conductor of asecond coaxial connector.
 13. The apparatus of claim 12, wherein thefirst coaxial connector and the second coaxial connector are mounted toa connection plate connected to the body; the coupling conductor spacedaway from the connection plate to allow clearance for the slotted tube.14. The apparatus of claim 12, wherein a mounting position of the firstcoaxial connector and the second coaxial connector is adjustable via aplurality of connector slots.
 15. The apparatus of claim 12, wherein amounting position of the connection plate onto the body is adjustablevia a connection plate slot.
 16. The apparatus of claim 10, furtherincluding a first end connected to the body coaxial with the first boreon the first side and a second end connected to the body coaxial withthe first bore on the second side.
 17. The apparatus of claim 16,wherein the first end and the second end are adapted for interconnectionwith one of a coaxial cable, a helically corrugated coaxial cable, and awaveguide.
 18. The apparatus of claim 10, wherein the coupling conductoris adapted to be adjustable laterally with respect to a longitudinalaxis of the first bore, within the slotted tube.
 19. The apparatus ofclaim 10, wherein a first end of the slotted tube is closed by a firstend plug and a second end of the slotted tube is closed by a second endplug.
 20. A method of varying a coupling factor between a transmissionline and a coupling conductor, comprising the steps of: positioning thecoupling conductor proximate an aperture in an outer conductor of thetransmission line; and covering the aperture to a degree providing adesired coupling factor.
 21. The method of claim 20, wherein theaperture is covered by planar movement of a gap plate.
 22. The method ofclaim 20, wherein the coupling conductor is positioned within a slottedtube; and the aperture is covered by rotation of the slotted tube. 23.The method of claim 20, further including adjustment of couplingdirectivity by adjusting a lateral position of the coupling conductorwithin a plane tangential to a longitudinal axis of the transmissionline.